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Fatigue and depression are among the most common and disabling symptoms of MS, being reported by over 90% and 40% of people with MS respectively. Cognitive problems and anxiety are also common.

Low sun exposure and low vitamin D levels are associated with an increased risk of developing MS, but their impact on these symptoms of MS is less well understood. Previous studies of vitamin D levels and fatigue and depression in people with MS have produced conflicting results, however these studies did not take into account sun exposure.

Researchers from the Menzies Institute for Medical Research, including Prof Bruce Taylor and Dr Ingrid van der Mei, together with their Dutch colleagues, set out to investigate the effects that sun exposure and vitamin D may have on fatigue and the neuropsychological symptoms of MS.

Their results have been published in the journal Acta Neurological Scandinavica (view the abstract here).

The researchers examined the personal sun exposure and blood vitamin D levels at several time points over a two and a half year period in 198 people with MS who took part in the Southern Tasmanian MS Longitudinal Study. Questionnaires were also completed at the same time points on fatigue, depression, anxiety levels and cognition.

The researchers found that there was no close association of vitamin D levels with levels of depression, but there did appear to be lower levels of depression in people whose blood level of vitamin D was over 80nM. However, when sun exposure was taken into account, this association was no longer statistically significant, suggesting that sun exposure has a more potent effect on depression which is independent of vitamin D synthesis.

Higher levels of sun exposure in the current season showed a consistent close association with a lower depression score. This was not affected by the level of physical activity.

Levels of fatigue did not show any connection to vitamin D levels or vitamin D supplementation. But again, higher sun exposure was significantly associated with lower levels of fatigue. Taking sleep quality into account did not change this association.

The researchers found no correlations between either sun exposure or vitamin D levels and anxiety or cognitive measures.

Because they had taken repeated measures over time, the researchers were able to use a statistical technique known as time-lag modelling to show that both depression and fatigue levels dropped following higher sun exposure. However, they could not completely rule out the possibility of reverse causality – that depression and fatigue prevented people with MS from going out in the sun.

Further research is required to understand the biological mechanism by which sunlight affects fatigue and depression, however the authors speculate that the release of anti-inflammatory chemical messengers in the skin which can affect mood, or the suppression of melatonin release by daylight may be involved.